In an article fabricated from molding resins it is sometimes desired to provide a means for mounting a device to the article. In those applications where the device may impose relatively large forces upon the molded article, it is frequently necessary to provide a suitably strong mounting means in the wall of the article to which the device can be mounted. Further, it is frequently necessary to provide a mounting means in the wall that will effect a leak-tight seal with the mounted device.
Articles molded from molding resins include hollow, single-piece, unitary tanks manufactured from thermoplastic or thermosetting molding resins or compounds. Devices, such as fittings, level sending mechanisms, and the like, may be mounted to the tank and, in some cases, project through the tank wall. Such devices are typically mounted with screws or bolts to the tank wall.
In the past, in certain applications involving vehicle fuel tanks, a mounting structure has been used that includes one or more metal inserts embedded in the tank wall for receiving mounting screws or bolts. Typically, each metal insert is completely embedded in the tank wall except for the opening of threaded bores in the insert structure which communicate with the exterior of the tank for receiving the mounting screws or bolts.
It is important that such metal insert structures be entirely encapsulated on the inside of the tank to prevent leakage of the tank contents. In the past, providing a leak-tight assembly around a metal mounting insert structure has been difficult to achieve. This is especially true in applications where a conventional fuel level sending unit is mounted to a fuel tank that is subject to vibration (e.g., a tank used for gasoline storage on industrial or agricultural equipment or vehicles).
Today, many fuel tanks for agricultural equipment are rotationally molded from a thermally cross-linkable high-density polyethylene resin, such as that resin sold under the trade name Marlex CL-100 by the Phillips Chemical Company. It is especially difficult to provide a leak-proof or leak-tight connection of a fuel level sending unit to an embedded, steel insert structure in the wall of such a fuel tank. Conventional steel mounting structures include those of the type with a generally flat annular ring having threaded bores therein and also include those of the type with an annular ring having projecting bosses uniformly spaced about the ring with a threaded, blind bore in each boss. Embodiments of the latter type of mounting structure are illustrated and described in the U.S. Pat. No. 4,023,257 to Wright et al.
As taught by the prior art, the tank wall is molded around the annular ring portion of the insert so that the entire ring portion is embedded in the wall. In the past, it was thought desirable to completely embed or encapsulate the mounting insert, except for the bore openings to the exterior of the tank. However, conventional rotational molding techniques with the thermally cross-linkable high-density polyethylene resins have not consistently produced leak-tight or leak-proof embedments.
The reasons for the failure of prior art mounting inserts to form a leak-proof embedment are believed to involve one or more parameters, such as the mounting insert shape, the mounting insert material, the molding resin compound, and the molding temperature. Although the reasons why conventional inserts frequently leak are not necessarily fully understood, it is believed that the molding resin does not flow around the mounting insert as completely as desired during the molding process. Cracks or voids are formed which lead from the tank interior to the interface between the molding resin and the insert. The voids can then communicate with the region around the bore openings at the exterior of the tank. This provides leakage paths for the tank contents. Capillary action, of course, can lead to, or add to, leakage through such paths.
It would be desirable to provide a metal mounting insert having a configuration adapted to be encapsulated in a molding resin and having at least portions that are adapted to be embedded in a wall of an article molded from the molding resin for subsequently accommodating the attachment of a device to the wall of the article.
Further, it would be desirable to provide a method for molding an article with the insert securely embedded therein as a leak-tight assembly. Also, it would be beneficial to provide a mounting insert made of a material which, when molded in a wall of an article, would contribute to the reduction or elimination of voids at the interface between the molding resin and the insert.
Also, it would be desirable to provide an insert in which threaded bores could be machined and in which the threaded bores would provide sufficient strength and resistance to stripping when threadingly engaged with fasteners for securing a device to the article wall.